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`cron` is a service used to run jobs on time-basis and is available on most UNIX-like operating system installations.
User `cron` jobs on O2 are run on a single server, `cronserver.rc.hms.harvard.edu`, and can be managed from any O2 login/compute node.
You don't have to do anything special to manage your `crontab` data on `cronserver`.
- The `crontab` command on all O2 systems (login/compute) will automatically connect to `cronserver` for you.
- The command is used in exactly the same way as on a standalone system.
- Run `man crontab` for complete documentation or run `crontab` with no arguments for a brief usage summary.
- To use a different editor than the default: `env EDITOR=vim crontab -e` (substitute the editor of your choice, e.g. emacs, pico, nano, etc. for "vim" in this example)
**General crontab help**
```
Usage:
crontab [options]
Options:
-e or --edit edit user's crontab
-l or --list list user's crontab
-r or --remove delete user's crontab
```
The `crontab` command will remind you about `cron` service as follows:
- For editing operations (`crontab -e`), a brief reminder and reference to this page will be included in the `crontab` editing buffer. This reminder will be removed after you save the file and before it is installed on `cronserver`.
- For non-editing operations (list, `crontab -l` and remove, `crontab -r`), a brief reminder and reference to this page will be printed on standard error. You can differentiate this text from the actual output of `crontab(1)`, which appears on standard output.
If you need to know the details of where `cron` jobs are run:
- You can find which O2 application server currently houses `cronserver` by running `ssh cronserver hostname` from any O2 system.
- However, you should not rely on `cron` administration or execution being housed on any specific system.
- If you need to refer to the system where `cron` jobs execute, be sure to use the role name `cronserver`.
- If you need something to execute at a specific time on a specific O2 system, use `ssh(1)` to initiate that command from a `cron` job on `cronserver`.
- We recommend that you [contact us](https://rc.hms.harvard.edu/#support) if you do this to make sure you are running your job in an appropriate location and that you are using service role names and application role users when needed.
**Finally, remember that any workloads with significant processing, memory, or storage I/O requirements should be run on an O2 compute node via the SLURM. Please refer to our
[Using Slurm Basic Guide](https://wiki.rc.hms.harvard.edu/display/O2/Using+Slurm+Basic)**
Please [contact Research Computing](https://rc.hms.harvard.edu/#support) if you have any questions about O2 `cron` service.